Conditioning of wood pulp for nitration



Patented Nov. 29, 1938 UNITED STATES want PATENT- OFFICE CONDITIONING F WOOD PULP FOR.

.NITRATION No Drawing. Application September 10, 1937,

Serial No. 163,258

Claims.

This invention relates to the conditioning of wood pulp for nitration. Its objective, generally stated, is to make possible the nitration of wood pulp in a fiuifed form resembling cotton linters 5 and the realization from pulp in such form of a high yield of nitrocellulose product exhibiting low acid retention and otherwise of commercially satisfactory character.

Because fiuffed wood pulp can be nitrated substantially as uniformly and readily as cotton linters-with the usual mixed nitrating acid in the usual nitrating equipment, it has heretofore been proposed to nitrate wood pulp in fiuffed form. However, when ordinary chemical wood pulps, such as bleached sulphite pulp, or even the usual refined wood pulps are fiuffed preparatory to nitration, the nitrocellulose yield is materially lowerand the acid retention materially higher than the values commercially acceptable. It might be noted that theracid retention value represents the amount of mixed nitrating acid retained by the nitrocellulose after it has been subjected to centrifugation as ordinarily for the extraction and recovery in substantially undiluted, reusable condition of the excess nitrating acid present therein after the nitrating operation. A high acid retention value is thus undesirable by reason of the fact that it bespeaks undue loss of the expensive nitrating acid in the washing 0r drowning of the centrifuged nitrocellulose.

Eflort has been made to improve the yield and lower the acid retention of nitrocellulose prepared fromlwood pulp in a form comparable 5 to that of cotton linters. One eifective method in this direction involves the shredding of the wood pulp in moist or humidified condition and drying the shredded pulp prior to nitration; The dried shreds thus prepared consist essentially of o bundles of fibers that have been rolled or twisted together during the disintegration of the pulp in the moist state; A mass of the dried shreds is relatively hard or harsh to the touch. -It is evidently the case that shredding of pulp within 45 certain limits of moisture content followed by drying does away. with the generation of fiber shredded pulp prior to nitration.

the predried pulp, shredding, and redrying the shreds also entails investment in drying equipment and the expense incident to drying the After considerable investigation and research on the problem of producing fiufled wood pulp capable of being nitrated with satisfactory yield and acid retention, we have discovered that it 20 is possible to prepare directly from dried wood pulp a fiuffed wood pulp product eminently satisfactory for nitration in respect of yield, acid 1 retention, and other results, provided that the wood pulp has undergone, prior to drying and fiumng in dried condition, treatment or exposure to strong caustic soda solution in sufiiciently cold or refrigerated condition to induce partial gelatinization or destruction of the pulp fibers, insomuch that the treated pulp fibers upon being washed and dried are of markedly greater density than the original pulp fibers, The gelatinizing action of the cold, strong caustic soda solution on the pulp fibers is evidently attended by such physical modification of the fiber wall orstructure that the pulp, upon being dried and fiuffed,

does not release fiber dust or fragments such as may well occasion undue loss of cellulose during nitration. Moreover, the fiuffed or individualized pulp fibers are evidently of such increased density or decreased wall porosity that they tend, after nitration, to throw off more readily theexcess nitrating acid during centrifugation. In any event, pulp subjected to the preliminary gelatinlzing treatment hereof withcold,strong caustic soda solution makes possible a fiufiing of the so-treated pulp in dried condition to a form resembling cotton linters and giving upon nitration eminently satisfactory results, insofar as concerns yield, acid retention, and quality of nitrocellulose product. It might be noted that the treatment of the pulp with cold, strong caus- 'tic soda solution favorably modifies the pulp for nitration from a chemical as well as physical standpoint, since such caustic soda solution tends cellulose constituents of the pulp. If desired, an oxidant, such as sodiumhypochlorite, may be added to the cold, strong caustic soda solution, especially when it is desired to prepare from Since the action or consumption of the oxidant at low temperature in transforming the pulp to a low-viscosity product is slow, it may be desirable to maintain the strong caustic soda solution containing'the oxidant in contact with the pulp in cold or refrigerated condition for a suflicient period of time to cause the desired gelatinizing action on the fibers and .then to heat the solution to foster rapid action or cbnsumption of the oxidant; or, vice versa, the oxidant dissolved in the caustic soda solution may first be substantially consumed or caused to act on the fiber at elevated temperature and the caustic soda solution then chilled or refrigerated to effect the desired partial gelatinization of the pulp fibers.

The invention hereof can be applied to various 4 kinds of wood pulp, including bleached sulphite pulp, bleached kraft pulp, and bleached wood pulps of high or supernormal alpha cellulose content, say, upwards of about 93%. For purpose of illustration, the invention hereof will be described in terms of a specific conditioning treatment applied to refined wood pulp, namely, bleached wood pulp of an alpha cellulose content of about 94%. Such pulp is treated typically with caustic soda solution of about 10% strength at about 5 C. for about an hour at a stock consistency of about 6% to 8%, whereupon it is washed substantially free of treating solution and,'if desired, treatedwith dilute sulphurous acid solution or the like so as to neutralize residual caustic soda and rewashed. In some instances, it may be desirable to treat the pulp as a dilute suspension in the cold, strong caustic soda solution, thereby avoiding the tendency of the fibers to clump or aggregate when formed into a thick suspension in cold, strong caustic soda solution. Thus, we may treat the pulp as suspensions of a fiber content of only about 0.5% or even lower, especially when the strong caustic soda solution used as, the suspending medium is maintained at about 0 C. during at least part of the fiber-treating period. The pulp fibers, which are preferably maintained in substantially uniform dispersion or suspension in the cold,

strong caustic. soda solution, as by stirring or agitation, undergo substantially uniform gelatinization or swelling. During the treatment, the physical change in the fiber structure or wall-is observable asthe thick, opaque pulp suspension in cold, strong caustic soda solution gradually acquires a semi-transparent appearance; and the partial destruction of the fiber structure or fiber characteristics is distinctly noticeable along side of a treatment of pulp performed at elevated temperature, say, 20 to 50 C., but otherwise under conditions similar to the treatment hereof. To the partial gelatinization or destruction of the fiber structure occurring in the cold, concentrated caustic soda solution is attributable the satisfactory nitration results hereof, for it has been established that satisfactory nitration results are not realized by fiuffing dried wood pulps previously refined in caustic soda solutions a to extract or dissolve pentosans and other hemitemperatures unaccompanied by noteworthy gelatinization or destruction of the fiber structure.

The washed, partially gelatinized pulp may be disrupted and the fibers individualized to form a dry fluff, it is preferable that the web or sheet formed from the pulp on the pulp drier undergo moderate or only light pressing or compaction at the wet end of the drier so as to issue from the drier in a loosely felted, dried condition consonant with its ready disintegration or resolution into aflnfiy mass of individualized pulp fibers resembling cotton linters. The-loosely-felted or sheeted pulp may be fed as pieces of suitable dimensions into a hammermill or other suitable disintegrator that serves to segregate the individual fibers into a fluffy or bulkymass reminiscent of absorbent cotton in its appearance and texture.

When flufled wood pulp products were prepared as hereinbefore described from bleached,

refined wood pulps and nitrated as ordinarily into nitrocellulose products, it was found that the yield and acid retention values of the nitrocellulose products were markedly superior to those realized in the nitration of dry-flufl'ed refined. wood pulps lacking the gelatinizing or conditioning treatment hereof. Indeed, the yield and acid retention values of nitrocellulose products prepared from the fiuifed wood pulp products hereof approximate those realized in the nitration of cotton linters. The following-table shows that whereas a bleached kraft pulp known on the market as "B-Cellate gave nitration results distinctly poorer than those realized in the case of cotton linters, the same pulp conditioned in accordance with the present invention can be nitrated with a yield and acid retention practically on a par with cotton linters.

' It might be noted that the nitrations in the foregoing examples were carried out under similar conditions to give a 12%-nitrogen nitrocellulose product, using mixed nitrating acid of 24% nitric acid content and carrying out the reaction at 40 C. for thirty minutes. The yield value given in the tables represents percent of nitro-'' cellulose product based on the bone-dry weigL of the pulp nitrated. The acid retention value given in the tables represents the weight in pounds of nitrating acid retained by each pound of bone-dry nitrocellulose after centrifugation of the nitrocellulose product.

It is possible to depart from the particular conditions of pretreating or} gelatinizing the wood pulp in cold, strong caustic soda solution while attaining the improved nitration results and other benefits of the instant invention. In any event, however, the pretreatment hereof involves the correlation of caustic soda solution of such strength and contact or admixture of the solution with the pulp at such temperature as to induce marked gelatinization of the pulp fibers or partial destruction of original fiber characteristics, as evidenced by increased density of the fiber wall after the pulp has been washed and dried. Generally speaking, the desired physical modification of the fiber, whether it be bleached sulphite pulp, bleached kraft pulp, or bleached refined wood pulp, can be effected at temperatures downwards of about 10 C. with caustic soda solutions of greater than about 8% strength. At temperatures of about 5 to 0 C. orv below 0 C., the desired physical modification of the fiber can be eflfected with caustic soda solution of about 8% strength, but the time of treatment may be shortened by using stronger solutions, say, of 10% to 18% strength. At a temperature range of about 5 to 10 C., it is necessary to use caustic soda solution of at least about 10% strength for the purposes hereof. The lower the temperature at which the caustic soda solution is applied and the stronger the solution applied at any particular temperature, the less the time necessary for efiecting the desired physical modification of the pulp fibers, but it is usually most expedient and economical to work with caustic soda solutions of about'8% to 18% strength at temperatures of about 0 to 10 C. Depending upon the particular strength of the caustic soda solution and the temperature at which it is applied, the time of treatment necessary may. vary from about 15 minutes to about 2 hours, after which the treated pulp may be washed either with cold or-warm Water and finally dried: Thedrying of the pulp may be carried to completion, that is, to produce a practically bone-dry product; or there may be some residual moisture, say, up to about 10%,

in the dried pulp product. It is to be understood,

therefore, that the expression dried pulp as used herein means either a bone-dry pulp product or one that contains the small amount of moisture, usually up to about 10%, present in an air-dry pulp so-called, since the improved results to be derived from the invention hereof are realized in fiufied pulps whose fiufiing has beenaccomplished in the substantial absence of moisture and in the presence of the small percentage of moisture indicated.

Solutions of other chemicals, such' as zinc chloride, alkali-metal thiocyanates, and strong solutions of sulphuric acid (parchmentizing strength) can also be used to eflfect a marked gelatinization of wood pulp or partial destruction of original fiber characteristics, whereupon the partially gelatinized pulp fibers may be washed substantially free from chemical gelatinizing solution, dried in interfelted sheet form or other economical'aggregated form, fiufied in dried condition, and finally nitrated. Irrespective of whether the pulp is in interfelted sheet form, in pressed, dried form, or in other aggregated, dried form, the aggregated, dried, pulp fibers can readily be disrupted, as in a hammermill or equivalent disintegrator, into a fiuffy mass of essentially individualized fibers resembling cotton linters, whereupon the flufiy mass can be nitrated and subjected to after-nitration treatments in essentially the same way as is customary in producing nitrocellulose from cotton linters.

Since the gelatinizing action on wood pulp of the various chemical solutions indicated is generally akin to that of cold, strong caustic soda solution, the present invention is to be regarded as embracing the use of such chemical solutions even thoughcold, strong caustic soda solution has been found to be particularly efiective and economical and is also desirable by reason of its beneficial chemical as Well as physical modification of the pulp. It is possible to add various chemicals to the caustic soda solution to enhance the gelatinizing or swelling action on' the pulp fibers, for instance, a small percentage of liquid carbon bisulphide, whose efiect is evidently to promote surface or incipient xanthation of the fiber; and this effect leads to regeneration of cellulose during the subsequent Washing, especially acid-washing, and high-temperature drying of the fiber, the regenerated cellulose thus contributing to the physical modification of" the fiber structure, including increased density of the fiber structure.

It is thus seen that by modifying the original structure of wood pulp fibers by partial chemical gelatinization of such structure, that is, gelatinization distinctly short of destroying such structure, it becomes possible subsequently to fiufi the wood pulp in substantially dry condition and thereby to produce a mass of wood pulp fibers in essentially individualized condition resembling cotton linters and giving upon nitration nitrocellulose-yield and acid-retention values such as are commercially acceptable. aware, we are the first to produce a fiuffy mass of substantially dry wood pulp fibers in essentially individualized condition resembling cotton linters, the original structure of the fibers having been sufiiciently chemically gelatinized, as by cold, strong caustic soda solution, so that such mass is capable of being nitrated with nitrocellulose-yield and acid-retention values approximating those realized in the nitration of cotton liners.

We claim:

1. In a process of producing nitrocellulose from wood pulp, thosesteps which comprise partially gelatinizing the wood pulp fibers in the form of a suspension in a chemical gelatinizing solution, as shown by change of the initially substantially opaque pulp suspension to a translucent or semitransparent appearance, washing the partially gelatinized pulp fibers substantially free from such solution, drying the washed pulp fibers in aggregated form to a moisture content less than about 10%, mechanically disrupting the aggregated dried pulp fibers into a fiufiy mass of essentially individualized fibers resembling cotton linters, and nitrating the fluify mass. 1

2. In a process of producing nitrocellulose from wood pulp, those steps which comprise partially gelatinizing the wood pulp fibers in the form of a suspension-in a chemical getalinizing solution, as shown by change of the initially substantially opaque pulp suspension to a translucent or semitransparent appearance, washing the partially gelatinized pulp fibers substantially free from such solution, drying the washed pulp fibers in interfelted sheet form to a moisture content less than about 10%, mechanically disrupting the interfelted, dried pulp fibers into a fiuiiy mass of essentially individualized fibers resembling cotton linters and nitrating the fluify mass.

3. In a process of producing nitrocellulose from Wood pulp, those steps which comprise suspending the Wood pulp in a caustic soda solution of suf- So far as we are ficient strength and at sufflclently low temperature to efiect a partial gelatinization of the pulp fibers, as shown by change of the initially substantially opaque pulp suspension to a translucent or semi-transparent appearance, washing the partially gelatinized pulp fibers substantially free from such solution, drying the washed pulp fibers in aggregated form to a moisture content less than about 10%, mechanically disrupting the aggregated dried pulp fibers into a fluflfy mass of essentially individualized fibers resembling cotton linters and nitrating the flufiy mass.

4. In a process of producing nitrocellulose from wood pulp, those steps which comprise suspending the wood pulp in a caustic soda solution of at least about 8% strength at temperatures downwards of about 10 C. for a sufiicient period of time to effect a partial -gelatinization of the pulp fibers, as shown by change of the initially substantially opaque pulp suspension to a translucent or semi-transparent appearance, washing the partially gelatinized pulp fibers substantially free from such solution, drying the washed pulp fibers in interfelted sheet form to a moisture content less than about 10%, mechanically disrupting the interfelted, dried pulp fibers into a flufiy mass of essentially individualized fibers resembling cotton linters, and nitrating the fiufiy mass.

5. In a process of producing nitrocellulose from wood pulp, those steps which comprise suspending the wood pulp in a caustic soda solution of about 8% to 18% strength at temperatures of about 0 to 10 C. for a sufficient period oftime to effect a partial gelatinization of the pulp fibers, as shown by change of the initially substantially opaque pulp suspension to a translucent or semitransparent appearance, washing the partially gelatinized pulp fibers substantially free from such solution, drying the washed pulp fibers in loosely interfelted sheet form to a moisture content less than about 10%, mechanically disrupt-,

ing the loosely interfelted, dried pulp fibers into a fiufiy mass of essentially individualized fibers resembling cotton linters, and nitrating the flufiy :0

mass.

GEORGE A. RICHTER. DOUGLAS H. MCMURTRIE. 

